1. Field of the Invention
The present invention pertains generally to the field of radio frequency (RF) power transistor devices and, more particularly, to gain and stability aspects of high power, high frequency RF power amplifier circuits.
2. Background
The use of RF power transistor devices as signal amplifiers in wireless communication applications is well known. With the considerable recent growth in the demand for wireless services, such as personal communication services, the operating frequency of wireless networks has increased dramatically and is now well into the gigahertz frequencies. At such high frequencies, laterally diffused, metal oxide semiconductor (LDMOS) transistors have been preferred for power amplification applications, e.g., for use in antenna base stations.
A typical power amplifier package employs paralleled LDMOS transistors formed on one or more dies, which is attached to a package substrate serving as both a heat sink and common ground element. Input (gate) and output (drain) package terminals, which are electrically isolated from the package, are connected to respective input and output terminals on the one or more die by many bond wires. Due to the very low input impedance of the LDMOS power transistors at high frequencies, the bond wires used to connect the package leads to the die cause a relatively significant impedance transformation. At times, this transformation is more than desired.
One early solution to this problem involved implementing multiple transformations from the die to the package leads, such as the coupling circuit described in U.S. Pat. No. 3,441,865 to Siwko. In devices using multiple transformations, however, a significant portion of the transformation occurs on the connecting device, which results in increased loss and bandwidth shrinkage.
Another approach to this problem is to use a parallel resonance matching circuit, as described in U.S. Pat. No. 5,315,265 to Wisherd et al. This method involves adding an inductor in parallel with each of the input and output common-source capacitances of a RF power amplifier. However, this method also suffers from problems of earlier solutions, including bandwidth shrinkage, because the transformation is done away from the power amplifier package.
In accordance with a main aspect of the invention, gain and bandwidth enhancement is effected by connecting a shunt inductance across the input, as well as the output (i.e., common element) terminals of a power transistor. The shunt inductance is chosen to be the conjugate of the common-source input capacitance of the transistor. In accordance with another aspect of the invention, a similar conjugate matching output circuit is provided to couple the power transistor""s common-source output capacitance to a load.
In one embodiment, gain and bandwidth enhancement of an LDMOS power amplifier is implemented by electrically connecting respective gate and drain terminals of the LDMOS transistor to grounded shunt caps via bond wires, the bond wires having an inductance chosen to be the conjugate of the common-source input and output capacitances of the transistor.
Other aspects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings.